In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup, supported at one end of a pickup arm, engages the spiral groove and includes a conductive electrode which establishes a capacitance with the conductive coating and the dielectric deposit of the disc record. When the disc record is rotated, the electrode-disc capacitance varies in response to the geometric variations in the bottom of the spiral groove passing beneath the signal pickup. The capacitance variations are converted to electrical signal variations by suitable signal processing circuitry coupled to the pickup electrode. The output signal of the signal processing circuitry may be coupled to a conventional television receiver for reproduction. The pickup arm is mounted on a signal pickup housing of the playback system. A system of the aforementioned type is described in detail in U.S. Pat. No. 3,842,194, issued on Oct. 15, 1974 to Jon K. Clemens.
Video disc systems of the aforementioned type generally utilize disc records having groove densities in the order of four to eight thousand groove convolutions per inch. A typical video disc record of this type may have a groove convolution spacing in the order of 3.5 microns. The fragile walls of relatively narrow grooves of the disc record cannot be dependably relied upon to pull the pickup arm assembly, around the pickup arm pivot support, across the entire recorded surface of the disc record. Also, in video disc systems utilizing the variable capacitor concept, it is desirable for accurate reproduction of the prerecorded signals that the signal pickup electrode maintain a substantially constant attitude in the spiral groove. Therefore, a radial feed drive mechanism is provided for traversing the signal pickup housing in proper time relationship with the radial motion of the signal pickup tip engaged in the spiral groove so as to continuously maintain the longitudinal axis of the pickup arm substantially tangential to the spiral groove at the point of engagement when the player is in a playback mode. Reference may be made to the copending U.S. application of F. R. Stave, Ser. No. 351,600, filed Apr. 16, 1973, entitled "VIDEO DISC PLAYBACK APPARATUS", and now U.S. Pat. No. 3,870,835, for an illustration of a suitable radial feed drive mechanism for providing the indicated radial motion.
Further, in the aforesaid video disc systems, adequate bandwidth for monochrome and color display is obtained by rotating the disc at a speed substantially higher (e.g., 450 rpm) than the conventional audio playback speeds (e.g., 33 and one-third rpm).
Additionally, in the above-mentioned video disc systems, groove engaging and low mass requirements of a signal pickup assembly result in a signal pickup unit which is extremely delicate (e.g., signal pickup width 2 microns, signal pickup depth 5 microns, signal pickup electrode width 2 microns, and signal pickup electrode depth 0.2 microns).
In the playback systems of the aforementioned type, it is desirable to effect a smooth landing of the signal pickup on the disc record disposed on the turntable for the following illustrative reasons. First, the disc record has fragile grooves (e.g., groove spacing 3.5 microns). Second, the signal pickup is delicate (e.g., signal pickup electrode depth 0.2 microns, and signal pickup electrode width 2.0 microns). Third, the disc record is rotated at a relatively high speed (e.g., 450 rpm). Fourth, the signal pickup electrode dimensions are critical due to a relatively high frequency video recording in the disc record grooves (e.g., 4 megacycles).
Further, it is desirable to provide a signal pickup lowering apparatus (1) which is relatively simple and rugged in construction, (2) which will provide a repeatable control of the signal pickup landing on the disc record, and (3) which will be suitable for relatively inexpensive mass manufacturing techniques.